The use of solid state light emitting devices (LEDs) for conventional lighting applications, such as vehicle light bulbs, interior and exterior lighting, and so on, continues to increase, due primarily to their expected useful life, and their efficiency.
In a conventional fabrication process, light emitting devices may be formed/grown on a first growth substrate or wafer, covered with a second, typically thinner, substrate or support material, and then the growth substrate is thinned or removed, effectively transferring the wafer-formed light emitting devices onto the second support material for subsequent processing. This subsequent processing may include the application of protective or functional materials, such as phosphor-embedded silicone, and the eventual dicing, or singulation, of the light emitting structures into individual light emitting devices comprising one or more of these structures.
FIG. 1A illustrates an example flow diagram for a conventional fabrication of a thin-film LED device with a phosphor coating, and FIG. 1B illustrates the structures formed during the corresponding fabrication stages.
At 110, light emitting device structures (LED dies) 101 are formed on a substrate (growth layer) 102, using techniques common in the art, which generally include forming at least an n-type layer, an active layer, and a p-type layer, and other layers, on a sapphire substrate or any suitable substrate, for example, silicon, silicon carbide, GaN, and so on. In this example, the LED dies 101 are structured to emit light through the ‘lower’ surface (typically the n-type layer) that is attached to the growth layer 102, and to have connections/pads for receiving power at the ‘upper’ surface (to and through the p-type layer), opposite the growth layer 102.
At 120, a secondary support structure 103 is attached to the upper surface of the LED dies 101. This secondary support structure may be a relatively thick sacrificial layer of removable material, or a film of removable ‘dicing tape’ on a frame that serves to hold the LED dies in place after the LED dies 101 are singulated.
At 130, the growth layer 102 is thinned or removed, to reduce interference to the light that will be emitted from the LED dies 101. To further facilitate light extraction from the LED dies 101, the light emitting surface 104 is finished, at 140, typically by roughening the surface 104 to reduce internal reflections.
At 150, a phosphor coating 105 is applied to cause a wavelength conversion of some or all of the light emitted by the LED die 101. In this manner, light output of a desired color is produced by the combination of wavelengths produced by the LED-phosphor combination. Obviously, if the LED emits light of the desired color directly, there is no need for this phosphor coating 105. Other coatings, such as protective coatings may also be applied.
At 160, the LED dies 101 with coating 105 are ‘diced’, or ‘singulated’, to provide individual devices that may subsequently be mounted on structures that facilitate handling and connection to a lamp or other illumination device. This dicing may be performed by laser or saw, the laser typically being preferred for its thinner kerf width, allowing for improved area efficiency by minimizing the required space between devices.
At 170, the secondary support material 103 is removed, allowing access to the connections to the LED die 101 on the now ‘lower’ surface, opposite the phosphor coating 105. If connection to the LED die 101 does not require access to the lower surface, or if the support material 103 provides the connections to the LED die 101, the support material 103 may not be removed.
When the LED dies 101 are formed on the growth substrate 102, the growth process and the combination of different materials, typically having significantly different thermal expansion characteristics, introduce stress within and between the LED dies. Accordingly, the growth substrate is purposely selected to be substantially rigid to avoid distortions, such as bowing, due to this stress.
However, the secondary support 103 is generally not as rigid, and when the growth substrate 102 is removed, at 130, these stresses cause distortions in the structure of the LEDs on the secondary support 103. These distortions will introduce curvatures in the streets between the rows and columns of the LED dies 101 that are used for dicing the dies 101. Accordingly, either additional steps must be taken to counteract this distortion, or allowances for this distortion must be made in the spacing between the LED dies 101, decreasing the area efficiency.
The typical kerf width of a laser cut is about ten microns, and, in the case of non-thinned growth substrate, the typical street width to accommodate for this kerf width and manufacturing tolerances is about thirty microns. In a structure formed by a six inch wafer that is thinned or removed, however, the distortion introduced by the growth stresses may be greater than thirty microns. Accordingly, either the yield will be decreased as the LED dies are mistakenly cut, or, the street widths must be significantly increased, often by a factor of two or more.
Additionally, some lamp assembly processes rely on the outer edges of the LED die to provide optical alignment with the light emitting surface; if a die is offset from the nominal center line of the street due to the distortion, the optical alignment will be similarly offset.